This invention relates generally to a method of cancer therapy sensitive to glycosphingolipid metabolism inhibition and, more particularly, to a chemotherapeutic method of treating cancer sensitive to glycosphingolipid metabolism inhibition by interfering with the metabolism of glycosphingolipids.
Sphingolipids are naturally occurring lipids in which the primary moiety is a long chain base, most commonly sphingosine. The long chain base consists of a fatty alkyl chain possessing a primary amine group at the C-2 position and two hydroxyl groups on the adjacent carbon atoms. While free sphingosine occurs at very low concentrations, most sphingosine molecules occur as amides, in which a long chain fatty acid is bound to the amino group of the base. The amides are called ceramides which serve as the metabolic precursor of all the complex sphingolipids and occur in tissues in readily detectable concentrations.
Ceramides possess a primary alcohol group at the terminal end of the long chain base (C-1 position) and a secondary alcohol group at the C-3 position. The secondary alcohol group is a free hydroxyl group in all known sphingolipids while the primary hydroxyl, in mammalian ceramide derivatives, is found bound in a beta-glycosidic linkage to a carbohydrate moiety or in an ester linkage to a phosphate moiety. The present invention is concerned with the former type of ceramide derivatives which are called glycosphingolipids.
The carbohydrate moiety that is directly attached in the glycosidic linkage in glycosphingolipids is either D-galactose or D-glucose. If only one sugar group is attached, the compound is called a cerebroside. More recently, cerebrosides have been more specifically termed glucosylceramides or galactosylceramides, depending upon their sugar moiety. The latter is found primarily in the nervous system, while the former appears to occur in all types of cells. The present invention deals particularly with glucosylceramide and its derivatives, hereinafter collectively referred to as "glucolipids".
Glucosylceramide occurs to some extent as a derivative in which D-galactose is linked in glycosidic linkage to the glucose; the derivative is called lactosylceramide or lactoside. Other glucolipids are formed from lactosylceramide by the sequential addition of other sugars, such as sialic acid, galactose, acetylglucosamine, acetylgalactosamine, and fucose. The products appear in the literature under nonsystematic names, such as globoside, hematoside, blood group substances, fucolipids, and gangliosides.
Because of the variety of linkages characteristic of sugars and the variety of enzymes responsible for linking them to one another, many different glucolipids occur in mammalian tissues. Most of these lipids are electrostatically neutral; however, the ones containing sialic acid, called gangliosides, can possess one or more negative charges, depending on the number of sialic acid moieties present. This type of glucolipid occurs in the grey matter of the nervous system at a relatively high concentration, but every type of cell contains a small amount of several types of gangliosides.
While the glucolipids have been known for many years, their low concentrations, lack of an easily quantitated group, and variety of structures have made them difficult to characterize and study. Hence, until recently, their existence and function have tended to be underplayed. However, more recently, there has been a growing recognition of the vital roles played by glucolipids in life processes. In addition to being important membrane components in animal cells, glucolipids appear to be intimately involved in tissue immunity and cell-to-cell recognition. Glucolipids, for example, mediate cell-to-cell recognition and communication by acting as distinguishing markers for cells from various organs of an animal. In addition, since the expression of glucolipids on the cell surface changes as the cell divides and differentiates, glucolipids may also be essential for the systematic growth and development of organisms.
The importance of glycosphingolipid metabolism is underscored by the seriousness of disorders resulting from defects in glycosphingolipid metabolism. For example, Tay-Sachs, Gaucher's, and Fabry's diseases, resulting from enzymatic defects in the glycosphingolipid degradative pathway and the accumulation of glycosphingolipids in the patient, all have severe clinical manifestations. Even more importantly, there is a growing body of evidence implicating glycosphingolipids, in particular, the glucolipids, in the cancer process.
For example, glucolipids have been implicated in the binding of cells to cementing proteins, such as fibronectin, and may therefore be involved in the penetrating, metastatic properties of cancer cells. They have also been found to occur as vital components of cell surface receptor sites which bind complex compounds, such as growth promoting factors which may be important in the runaway growth of cancer cells. Additionally, the glucolipid composition of cultured cells changes when the cells are infected with a virus or induced to multiply rapidly. Gangliosides have produced marked proliferation or growth of cells in vitro and in vivo.
Cancer cells also seem to have a high rate of metabolic activity of the glucolipids. Human leukemic cells have been found to have triple the normal level of glucosidase, the glucosylceramide degradative enzyme. Studies have shown ten times the normal specific activity of glycosphingolipid glactosyltransferase in a rathepatoma. A human tumor was also found to contain a glucolipid-forming enzyme, an acetylglucosamine transferase, foreign to normal cells.
The injection of glucosylceramide into mice has been shown to produce marked, rapid stimulation of the growth of the liver (which preferentially absorbs the lipid). In the case of mice bearing Ehrlich ascites carcinoma cells, the injection of glucosylceramide resulted in a greater than 50% increase in the number of cancer cells. Recent studies have also indicated that patients with Gaucher's disease, which results from an accumulation of glucosylceramide, have an unexpectedly high incidence of leukemia and other B-cell proliferation disorders.
Tumors, as they progress from slightly malignant to intensely malignant, exhibit marked changes in their assortment of glucolipids. These glucolipids from tumors have been found to interfere with the ability of lymphocytes to proliferate. This may explain the lack of effectiveness of the cancer patient's immunological protection mechanisms. However, the injection of antibodies against specific glucolipids (prepared in mice) has proved very effective in patients with melanoma.
Significantly, researchers have been finding glucolipids in tumors which do not occur in the tissue of origin (although the lipids may occur to a small extend in other normal tissues). At an accelerating pace, scientists have been finding that some of the glucolipids have never been seen before in any normal tissue. In other words, it appears that many or all tumors have the ability to produce or to accumulate new glucolipids, normally foreign to human beings. It is also important to note that all of these substances are glucolipids formed enzymatically from glucosylceramide.
A significant body of evidence shows that glycosphingolipids, in particular, glucosylceramide and its derivatives, i.e. glucolipids, play an important role in controlling the proliferation and metastasis of at least certain types of cancerous cells. This indicates that cancer cells may be peculiarly sensitive to interference with glycosphingolipid metabolism. Thus, an object of the present invention is to provide a method of chemotherapeutic treatment of cancer sensitive to glycosphingolipid metabolism inhibition by interfering with glycosphingolipid metabolism. A further object of the present invention is to provide a method of chemotherapeutic treatment of non-malignant conditions caused by uncontrolled cell proliferation such as, for example, benign tumors.